Rifles belonging to the M16 family, such as M16, AR-15, and their variants, are gas-operated, selective fire, magazine fed rifles. A unique feature to these rifles is the direct impingement gas-operation system used to cycle the firing action. In traditional gas-operated reloading mechanisms, a portion of the high-pressure gas from a fired cartridge is used to operate a piston and rod arrangement which prepares the firearm for the next firing cycle. One drawback to a traditional gas-operated mechanism is the added complexity and weight of the piston and rod. Further, the use of a piston and rod arrangement may increase the recoil of the firearm. In a direct impingement gas-operation system as found on M16-type rifles, the piston and rod arrangement is removed, and high-pressure gas is ported from the barrel directly back to the firing mechanism to prepare the firearm for the next firing cycle.
More specifically, M16-type direct impingement systems work as follows. Upon firing a cartridge, high-temperature, high-pressure gas follows the exiting projectile down the barrel. The barrel includes a small port, coupled to a tube which runs parallel to the barrel. A portion of the gas from the fired cartridge travels into the port and tube. The other end of the tube enters the upper receiver of the rifle, where it enters the bolt carrier key, or gas key. The gas key is coupled to the bolt carrier, and the gas key includes an internal port to allow the high-pressure gas to flow into the bolt carrier. Once the gas enters the bolt carrier, it expands, and the pressure from the gas urges the bolt carrier away from the barrel. However, the bolt initially resists the bolt carrier's movement, as the bolt is temporarily locked to the barrel for firing. The urging of the bolt carrier acts on a cam pin, which causes the bolt to rotate so at to be unlocked from the barrel. The bolt carrier and bolt then translate away from the barrel, extracting the empty cartridge, and compressing the buffer return spring located in the buttstock. The forward movement of the bolt and bolt carrier off the return spring first strips a fresh cartridge from the ammunition magazine and, on the final stage of the movement, rotates the bolt to lock it into the barrel, ready for firing.
A forward assist device is optionally used on M16-type type rifles. This is a back-up device that engages serrations on the right side of the bolt carrier to manually push the bolt carrier forward in the event the force from the return spring is insufficient to push the bolt carrier forward into a locked position. A common reason for the return spring not being able to move the bolt carrier into a locked position is from fouling or debris inside the upper receiver or breech. As the rifle will not fire unless the bolt is locked against the barrel and the bolt carrier is in its forwardmost position, the forward assist is necessary to ensure normal operation of the rifle in all conditions. The bolt carrier and the bolt itself may be chrome-plated to resist debris build-up. The “T”-shaped charging handle is located at the rear of the receiver, above the buttstock, and does not reciprocate when gun is fired. The charging handle is configured to hold the bolt in an unlocked position for cleaning or maintenance.
The firing and locking mechanisms in M16-type rifles is contained primarily in the upper receiver, which is removably coupled to the other components of the firearm such as the lower receiver, buttstock, and barrel.
Some advantages of direct impingement systems over other gas-operated systems is increased simplicity, lighter weight, reduced recoil, smoother operation, and reduced cost. One disadvantage of direct impingement systems is that the breech of the firearm may become fouled more easily, as the gasses act directly on the bolt. The gasses that operate the firing and locking mechanism contain impurities such as carbon and metal that become deposited on the moving parts of the firing mechanism, which can lead to misfires and unreliable operation. Frequent cleaning of the breech, the bolt, the bolt carrier, and the firing pin is necessary to remove carbon and metal deposits.
Even with frequent cleaning, trouble-free operation of a direct impingement firearm is not guaranteed. Misfires still occur even with regular cleaning, due to external debris such as sand, mud, snow or water entering the breech. Each time a round is fired, the bolt unlocks and then re-locks, providing an opportunity for debris to enter. Operating conditions experienced during combat can expose the firearm to such external debris, at a time when reliable operation of the firearm is crucial.
A further disadvantage of M16-type rifles is the forward-placed, rotary locking mechanism. The multiple moving parts and complexity of this arrangement can lead to unreliable operation.
Previous solutions to the reliability problem of direct impingement firing mechanisms have been inadequate due to their complexity, cost, or ineffectiveness. Accordingly, there is a need in the industry for a simple, effective and reliable gas operating system.